A) It is a derived spread, but there is no real legging risk as the orders allowed are all IOC(immediate or cancel). So for example you place an order to buy 10 lots of Jan/Feb Nifty spread at Rs 50, only if it is available at 50 it gets executed, otherwise the order gets cancelled. So yeah that could mean an opportunity cost, so yeah it is more from a filling point of view. Cost remains the same, if you execute through the spread order window, or individually.
B) Have you checked our brokerage calculator(http://zerodha.com/brokerage_calculator). So basically the costs are Brokerage, Exchange and govt charges. Let me give you an example, say 20 lots of Nifty (500 Nifty) you put up a spread. Say you are buying 20 lots of Jan and shorting 20 lots of Feb. You then reverse this. So the round trip is 4 trades. So the brokerage is just Rs 80 ( 20×4) that we charge. All the statutory charges, you can find on the calculator.
Let me see if someone is selling the historical price movement of spread. You could probably put a query with esginal.

Unlike a normal client, we as members of the exchange can’t take the funds directly from an NRI. All your funds has to sit with what is called a custodian. This custodian, charges you over and above thebrokerage that we charge for providing the services (around Rs 300 to Rs 500 per crore). Also, since unlike a normal client we don’t have access to your funds, we have to put an incremental effort to support a custodial client (daily correspondence with the custodian to get your margins and positions, get a confirmation from the custodian everytime a trade happens), hence the brokerage charged is also higher. (We charge Rs 200 per executed order).
The taxes remain the same.